1. Field of the Invention
The present invention is related to silicon nitride ceramics and more particularly to a method for controlling the microstructure of a silicon nitride ceramic to vary depending on the region therein and to a method for manufacture thereof.
2. Description of the Background Art
Silicon nitride (Si.sub.3 N.sub.4) ceramics are one of the leading candidate materials for high-temperature engineering applications because of their excellent thermo-mechanical properties. It is well known that densification of Si3N4 is achieved by liquid-phase sintering using metal oxides such as Y.sub.2 O.sub.3, Al.sub.2 O.sub.3, and rare-earth oxides as sintering additives. During sintering, these additives form an eutectic liquid around 1500.degree. C. together with SiO.sub.2 present at the surface of the Si.sub.3 N.sub.4 starting powder. This liquid phase promotes the transformation from .beta.-phase as well as the densification. Due to this liquid phase sintering process, a Si.sub.3 N.sub.4 sintered body has an unique microstructure; large elongated grains randomly dispersed in fine matrix grains.
Elongated grains in the densified specimen play an important role in determining the mechanical properties of Si.sub.3 N.sub.4. During the fracture process, these elongated grains resist crack propagation effectively, like whiskers or fibers in composite materials, resulting in the increase in fracture toughness of the material. However, excessive growth of the elongated grains tends to decrease the stength of the Si.sub.3 N.sub.4 due to the formation of large defects around the grains.
To increase the strength and the fracture toughness of Si.sub.3 N.sub.4 at the same time, it is necessary to combine these two distinct microstructures in one material. One approach for this combination is fabricating a layered composite material. Strength of a ceramic is determined by the microstructure of the surface material because fracture is initiated from flaws at the surface. On the other hand, the fracture toughness is a property of the bulk because it represents the resistance of the material against crack propagation after formation of the crack. Therefore, to increase both the strength and the fracture toughness of Si.sub.3 N.sub.4 at the same time, the microstructure of the surface layer should be fine and homogeneous and that of the bulk layer should be large and elongated. By making a layered composite, in which a layer with a fine microstructure is placed on the top of a bulk layer having large elongated grains dispersed in fine matrix grains, both the strength and the fracture toughness of a Si.sub.3 N.sub.4 ceramic have been improved.
A disadvantage of this technique is that this type of material can only be fabricated by using a hot-pressing process. This fabrication technique restricts the shape of the material to only the simplest ones. In addition, the hot-pressing process is very expensive to implement and is not suitable for the mass production of materials.